A display device using an organic electroluminescence (EL) device is known as an example of a display device using a current-driven light-emitting device. An organic EL display device using such a self-luminous organic EL device does not require a backlight necessary for a liquid-crystal display device and is most suitable when a lower-profile device is desired. Since there is no limit on a viewing angle, organic EL display devices are expected as next-generation display devices to be put to practical use. An organic EL device used in an organic EL display device is different from a liquid-crystal cell which is controlled by a voltage applied to the liquid-crystal cell in that luminance of each light-emitting device is controlled by a value of a current flowing through the light-emitting device.
In an organic EL display device, organic EL devices each constituting a pixel are generally disposed in a matrix. An organic EL display device which has an organic EL device provided at each of intersections of a plurality of row electrodes (scanning lines) and a plurality of column electrodes (data lines) and drives the organic EL devices by applying a voltage corresponding to a data signal between a selected one of the row electrodes and the plurality of column electrodes is referred to as a passive-matrix organic EL display device.
There is another organic EL display device which has a switching thin film transistor (TFT: Thin Film Transistor) provided at each of intersections of a plurality of scanning lines and a plurality of data lines, and a gate of a driver is connected to each switching TFT. A data signal is provided from a signal line to the driver by turning on the switching TFT through a selected one of the scanning lines. An organic EL display device which drives an organic EL device using the driver is referred to as an active-matrix organic EL display device. An active-matrix organic EL display device can cause organic EL devices to emit light until a next scanning (selection) operation, unlike a passive matrix organic EL display device of which organic EL devices connected to respective row electrodes (scanning line) emit light only during a period when the scanning line is selected, and thus even an increase in the number of scanning lines does not invite a reduction in luminance of the display. An active-matrix organic EL display device can thus be driven at a low voltage, which allows a reduction in power consumption.
For example, PTL 1 discloses a circuit configuration of a pixel unit included in an active-matrix organic EL display device.
FIG. 22 is a diagram showing a circuit configuration of a pixel included in a display device described in PTL 1, and a connection with circuits around the pixel. A display device 100 shown in the diagram includes a pixel array unit in which pixels 100a are disposed in a matrix, and a drive unit that drives the pixel array unit. In the diagram, only one of the pixels 100a included in the pixel array unit is shown for convenience. The pixel array unit includes: scanning lines 102 disposed for respective rows; data lines 101 disposed for respective columns; the pixels 100a disposed in rows and columns at the intersections of the scanning lines 102 and the data lines 101; and power supply lines 110 disposed for respective rows. In addition, the drive unit includes a horizontal selector 103, a write scanner 104; and a power drive scanner 105.
The write scanner 104 sequentially supplies control signals to the scanning lines 102 in respective horizontal cycles (1H) and line-sequentially scans the pixels 101, one row at a time. The power drive scanner 105 supplies a variable power voltage to the power supply lines 110 in time with the line-sequentially scanning. The horizontal selector 103 switches between the data voltage which is a video signal and a reference voltage in time with the line-sequentially scanning and supplies the voltage to the columns of the data lines 101. Each of the pixels includes: a drive transistor 111; selection transistors 112a and 112b; an organic EL device 113; and a capacitor 114. Each of the selection transistors 112a and 112b is a thin film transistor composing a gate group 112. The drive transistor 111 and the organic EL device 113 are connected in series between the power supply line 110 and a reference potential Vcat (a ground potential, for example,). With this configuration, the cathode of the organic EL device is connected to the reference potential Vcat and the anode is connected to the source of the drive transistor 111, and the drain of the drive transistor 111 is connected to the power supply line 110. In addition, the gate of the drive transistor 111 is connected to a first electrode of the capacitor 114 and one of the source electrode and the drain electrode of the selection transistor 112b. Furthermore, a second electrode of the capacitor 114 is connected to the anode of the organic EL device 133.
In addition, one of the source electrode and the drain electrode of the selection transistor 112a that forms the gate group 112 is connected to the other of the source electrode and the drain electrode of the selection transistor 112b. In addition, the data line 101 is connected to the other of the source electrode and the drain electrode of the selection transistor 112a. The gates of the selection transistors 112a and 112b are connected to the scanning line 102.
In the above-described configuration, the power drive scanner 105 switches the power supply line 110 from the first voltage (high voltage) to the second voltage (low voltage) with the data line 101 being at a threshold detecting voltage. Likewise, the write scanner 104 raises the voltage of the scanning line 102 to a high level with the data line 101 being at the threshold detecting voltage to bring the selection transistors 112a and 112b into conduction, and apply the threshold detecting voltage to the gate of the drive transistor 111. Next, the power drive scanner 105 switches the voltage of the power supply line 110 from the second voltage to the first voltage to cause the capacitor 114 to hold the voltage corresponding to the threshold voltage of the drive transistor 111, in a correction period before the voltage of the data line 101 switches from the threshold detecting voltage to the data voltage. Next, the write scanner 104 changes the voltages of the selection transistors 112a and 112b to a high level and causes the capacitor 114 to hold the data voltage. That is, the data voltage is added to the voltage corresponding to the threshold voltage of the drive transistor 111 held previously, and written in the capacitor 114. Then, the drive transistor 111 is supplied with a current by the power supply line 110 at the first voltage and causes a drive current corresponding to the voltage that is held to flow in the organic EL device 113.
As described above, the write scanner 104 turns the gate group 112 on and off, thereby writing and holding the data voltage. The configuration in which two selection transistors are connected in series, as in the gate group 112, is referred to as a double-gate configuration. The double-gate configuration contributes to doubling the off resistance of the gate group 112. Furthermore, even when one of the selection transistors causes off-leakage, the other selection transistor curbs the off-leakage, reducing the off-leakage current approximately by half.
PTL 1 states that the above-described double-gate configuration allows accurately writing luminance information into a pixel and providing a display device with a high image quality without causing variation in the luminance of the organic EL device 113.
In addition, there is a known method of determining whether or not any of the selection transistors 112a and 112b included in the gate group 112 and the capacitor 114 is faulty, that is, a method of determining pass or fail of the pixel 100a. As shown in FIG. 23, a charge is written in each of the pixels 100a, and the charge is sequentially read from each of the pixels 100a upon completing the writing. Then the written value and the read value are compared, thereby determining the pass or fail of the pixel 100a. 
More specifically, when the written value and the read value are identical, it is found that none of the selection transistors 112a and 112b and the capacitor 114 is faulty, in other words, the pixel 100a is acceptable. Furthermore, when the written value and the read value are different from each another, it is found that any of the selection transistors 112a and 112b and the capacitor 114 is faulty, in other words, the pixel 100a is unacceptable.